Dams in California

Dams play a crucial role in California’s water management.
California relies heavily on nearly 1,500 reservoirs for managing water supply. The state’s dry summers and frequent droughts require abundant storage to meet water demands. In an average year, roughly 70% of the water used by cities and farms comes from rivers, and dams play a key role in regulating this supply. Most dams and their reservoirs are owned and operated by local agencies and private companies. But state and federal agencies manage 240 large reservoirs that account for 60% of the state’s storage capacity.

Dams are operated to meet multiple objectives …
The state’s dams provide multiple benefits in addition to storing water for cities and farms. Dams generate 15% of the state’s electricity supply on average. Some are operated to capture runoff from winter storms, reducing flood risk on the state’s large floodplains—this is essential for cities in the Central Valley and Southern California. Dams support a large reservoir-based recreation industry. And in California’s highly managed water system, flow releases from dams are essential to meeting the habitat needs of fish and wildlife.

Dams vary in size and ownership

SOURCE: US Army Corps of Engineers National Inventory of Dams.

NOTE: Local dams include those operated by local agencies and private companies, such as power utilities.

… but these objectives are often in conflict.
Many large multipurpose dams are operated with conflicting goals. For example, to manage floods, operators must release enough water to create space in reservoirs for winter floodwaters, which increases the chances that reservoirs will not be full in spring. Over the summer, when recreation demands are highest, reservoirs are drawn down rapidly to meet water and hydropower demands. Finally, many dams are required to conserve and slowly release cold water—which collects at the bottom of reservoirs—to support downstream salmon and steelhead habitat. Managing these trade-offs is becoming increasingly challenging as California’s climate warms and precipitation becomes more variable.

Many dams need infrastructure and operational upgrades.
Half of California’s dams are at least 50 years old. Most dams were designed—and are currently operated—based on simplistic assumptions about hydrology and earthquakes. More than 90 need major upgrades to better handle large floods or withstand earthquakes. Dam operations also need to be updated to work with improved weather forecasting technology and account for a changing climate. The 2017 Oroville Dam crisis—which led to the evacuation of nearly 200,000 downstream residents—highlighted many of these issues.

Most of California’s dams were built in the mid-20th century

SOURCES: US Army Corps of Engineers National Inventory of Dams, California Department of Water Resources California Data Exchange Reservoir Information.

NOTES: Figure does not include 86 dams for which the year of construction is unknown. The five largest dams in the state are listed by name. The number of dams includes main reservoir dams, but also spillways, dikes and other auxiliary dams.

New dams can improve flexibility, but costs are high.
Increased surface storage would improve supply reliability in some regions and allow greater flexibility in operations. State bond funding is available to help pay for more storage, and there are many proposals to build new dams or expand existing facilities. But California already has dams at the best locations and new dams will be costly to build and operate. The state also has opportunities to increase storage in its groundwater basins, in some cases at relatively low cost. Coordinating surface and groundwater operations—principally by moving water out of reservoirs and into aquifers during wet periods—can increase the total amount of water stored.

Some dams are ripe for removal.
Reasons to remove a dam include high environmental costs, earthquake safety hazards, and reduced benefits—for instance, when reservoirs fill with sediment, they lose their capacity to store water. Over the past 30 years, 36 small dams have been removed in California. The 2015 breaching of San Clemente Dam on the Carmel River was the largest dam removal in state history. Several other large dams have been targeted for removal, including Matilija Dam in Southern California and four aging hydropower dams on the Klamath River in Northern California.